In recent years, the flow of reduction in size and weight and high performance of electronic instruments rapidly advances. For this reason, it is also required to make electronic parts to be used in an electronic instrument small in size. In order to cope with this, realization of a high density of printed wiring board becomes a big problem. The layer structure of a multilayered printed wiring board has a structure in which a planar conductive layer (wiring pattern) and an insulating layer are alternately laminated. Lamination of such a wiring pattern is advantageous in realizing a high density.
Hitherto, a 50 μm-thick rolled copper foil has been used because it is cheap and good in handling property. Following the high density of a wiring pattern, a 35 μm-thick rolled copper foil has also been recently provided. Further, though the film thickness of not more than 10 μm is demanded, it is the actual situation that it is likely difficult from the standpoints of handling property and operability to provide a copper thin film having such a film thickness by a pressure roll.
Then, in place of the rolled copper foil, there is developed a system in which an extremely thin conductive layer is formed on a resin film by the sputtering or electroless plating method, a copper thin film of about 10 μm is formed by cheap electrolytic copper plating, and this copper thin film is then transferred onto a substrate. This technology of forming a copper thin film on a resin film is a technique that is widely employed for flexible printed wiring boards or thin film copper films for TAB (tape automated bonding) tapes. Recently, this technology is also applied as a technique in which a copper thin film can be cheaply and simply formed and transferred onto a desired substrate to produce a multilayered printed wiring board.
In the multilayered printed wiring board, with respect to a conventional technique of insulating layer prepregs partitioning the planar conductive layer, there is generated a technical problem because of its high density. Hitherto, connection (that is, interlaminar connection) of upper and lower circuit patterns (conductive layers) has been carried out by forming a via hole (forming a hole on an insulating layer) by drilling and forming a metal plating in that portion. However, the formation of a via hole by such drilling is difficult in making the size of the via hole small and is not suitable for realizing a high density.
On the other hand, as one method of realizing a high density of printed wiring board, the build-up method is recently watched. Its characteristic feature resides in the matter that the formation of a hole for interlaminar connection is carried out by forming a fine hole (via hole) between the layers using a photosensitive insulating film in place of the conventional drilling method.
For example, a specific example of the method of forming interlaminar connection by a via hole of this photosensitive insulating film is described in JP-A-4-148590. In this method, a photosensitive insulating resin layer is provided on a first circuit pattern having a wiring pattern of a copper layer formed on a substrate, a via hole is formed by photolithography (imagewise exposure and development), and its surface is then subjected to a chemical roughing treatment with a potassium permanganate solution. In this case, the chemical roughing treatment is carried out for the purpose of enhancing an adhesive strength between the insulating resin layer and an electroless plated copper layer (and an electrolytically plated copper layer formed thereon). It is considered that fine irregularities are formed on the resin surface, whereby the adhesive strength is enhanced due to a so-called anchor effect. Copper is attached onto the insulating resin layer having a via hole formed thereon by electroless plating. In this way, copper is also attached onto the via hole and the first circuit pattern (copper layer) in the bottom of the via hole, whereby a copper layer for forming a second circuit pattern to be connected to a first wiring pattern is formed. This copper layer is patterned to form a second wiring pattern. Then, the desired number of wiring patterns can be formed in the same way.
However, according to the method described in JP-A-4-148590, that is, the method of forming irregularities on the surface of the insulating resin layer by a chemical treatment (surface roughing treatment), since the use of potassium permanganate that may possibly adversely affect the environment is essential in forming irregularities on the surface, such a method is not preferable in view of safety or environment.
On the other hand, for the sake of enhancing the adhesive strength between an insulating resin layer and an electroless plated copper, JP-A-63-126297 describes a method in which particles soluble in an acid or an oxidizing agent solution are dispersed in a photosensitive insulating resin, and after curing the photosensitive insulating resin, the dispersed particles are dissolved with a specific chemical (oxidizing agent) made of chromic acid, a chromate, a permanganate, ozone, etc. to form irregularities on the surface of the photosensitive insulating resin layer, thereby enhancing the adhesive strength to e metal film by means of electroless plating.
However, even this disclosed method is not preferable in view of safety or environment because the treating agent to be used is an oxidizing agent or a strong acid. Further, according to this method, though the particle-containing photosensitive insulating resin layer is formed by coating, in the case where the particle-containing photosensitive insulating resin layer is provided on an insulating substrate in the lamination (thermo-compression bonding) system that is simple in works of the step and less with respect to defects in the formed layer in comparison with printing or coating, since the particles are dispersed in the photosensitive insulating resin, defects such as incorporation of bubbles onto the insulating substance are liable to be caused, resulting in a problem of difficulty from the standpoint of practical use.
In order to cope with these technical demands, there is developed a technique of forming a previously roughed thin film metal layer and releasing and transferring it. For example, JP-A-2000-309898 and JP-A-2001-68804 disclose that for the purpose of enhancing the release property between a support and a transfer metal thin film, a specific compound such as thiocyanuric acid and nitrogen-containing organic compounds is used in a joining interface. JP-A-10-146915 discloses an adhesive-attached copper foil obtaining by continuously coating a curable resin composition that is cured by heat or active energy rays and heat on a surface-treated extremely thin copper foil having been temporarily adhered onto a carrier and optionally subjecting to B-stage curing (semi-curing) with heat. JP-A-10-261870 discloses a material for multilayered printed wiring board by the build-up method in which an insulating adhesive layer in the semi-cured state is provided on the roughed metal foil surface.
Further, JP-A-2000-141542 discloses a copper foil comprising a resin film having an electroless copper-plated layer and a roughed electrolytically copper-plated layer laminated in this order on the surface thereof. However, in order to impart good adhesion to an insulating layer in the next step, it is essential to perform a roughing step in the side coming into contact with the support. A technique for realizing not only good release property but also cheap, simple and good adhesion has been demanded.